Buying a parts bike but confused

[Littleston101]

I have a 2003 CR125. Needs a new top end. The cylinder is scratched and the piston is charred. It was already bore twenty over....Well a new cylinder is $330. and a new piston is $50. boring out the cylinder and going up a size on the piston is $220....I found a parts bike on craigslist. it is a 2002 cr125. From what I understand, the 2002 cr125 has the same parts as the 2003? the top end is the same? well this bike is in good condition. the guy wants $400 for it. It has:

Sunline Levers, Renthal Bars, Pro Taper Clamps, New Tires, FMF Pipe, and the rims and stuff are good.

The pipe on my bike is rough and there is surface rusting. My front rim is bent as well. I figured I could use this as a parts bike. Well I went and look at it today, and tore off the cylinder head....looked at the piston and it seemed fine. I also looked at the cylinder on the top (inside) and it seeemed ok too. I figured I could just bolt it right on my bike and be good to go. The bike kicks alright. And has good compression. I don't understand why it won't run though......... I tried to unbolt the cylinder and take it off to look at it but I couldn't fit a wrench in the bolt that is inside the powervalve. Well when I unbolted the powevalve, it was all greasy and black.

The kid said it had a fresh rebuild and he was excited and went to ride it and put some crappy 2-cycle gas station oil in it. and ran a 40:1 ratio on a fresh top end.....I feel like this burnt out the spark plug but I dont understand why the powervalve is so nasty looking. Is this normal?

 

[whenfoxforks-ruled]

I guess it depends on who is working and riding it, if it is normal or not. I would say no, and you really need a shop manual for 1 of those bikes, if not both. You also, can not rely on used gaskets, rings and surely not the cir clips. And by dropping the premix ratio to 40:1, you are flooding the engine with even more gas! There is no way to visually inspect a cylinder and determine if it is useable. Unless it included some actual measuring? The first cylinder has a steel liner installed in it, does the second one also? You can verify that with a magnet, the stock cylinder is plated and not magnetic. As far as compatible between 02 and 3, it may bolt up, but will it work? Vintage Bob

 

[slickpuss]

Some people throw a new top end in without cleaning up the powervalve assembly. Thats probably why its all gummed up. Its best to disassemble it and clean it well.

The 02 and 03 should share the majority of the top end parts, that shouldnt be an issue. A new cylinder is only 250 from service honda. Compared to the cost of maintaining a 4 stroke you should have just bought the new cylinder and top end and counted as the cost of upkeep.

I just had to replace my 125's cylinder last year myself and went through all the options, and a new cylinder is the best option in my opinion.